There has been a conventionally-known circuit which incorporates an impedance element such as a capacitor, a resistance, an inductor, and a transmission line, with a variable capacitance diode, to change impedance, resonant frequency or the like and thereby provide desired circuit characteristics, thus allowing for switching of frequency bands and decreasing characteristic changes depending on use environment.
There has also been a circuit which is low in loss even at a high frequency with use of, instead of the variable capacitance diode, a variable capacitance element having a thin-film dielectric layer with dielectric constant varying with a direct-current voltage.
Furthermore, there has been disclosed a circuit having excellent characteristics such as a low loss even at a high frequency, power handling capability, and small distortion, with use of a variable capacitance element unit in which a plurality of variable capacitance elements are connected in parallel with respect to direct current and in series with respect to high frequency (refer to Japanese Unexamined Patent Publication JP-A 2005-101773).
However, the circuit employing the variable capacitance diode is allowed to be used only in a receiver or receiving circuit adapted for low power because the variable capacitance diode is low in power handling capability and experiences large distortion due to capacitance nonlinearity, that is, the circuit has problems of being not allowed to be used in a transmitter or transmitting circuit adapted for high power and furthermore of having a high loss at a high frequency.
In the circuit employing the variable capacitance element having the thin-film dielectric layer with dielectric constant varying with a direct-current voltage, the circuit loss can be lower because the loss in the variable capacitance element can be lower even at a high frequency, but the variable capacitance element has its capacitance varying even with a high frequency voltage and therefore has such a problem that distortion including waveform distortion and intermodulation distortion is larger in the case where the high frequency voltage is high.
In order to reduce the above distortion, high-frequency field strength of the variable capacitance element needs to be lowered to reduce the capacitance variation caused by the high frequency voltage, and to achieve this, it is effective to increase a thickness of the dielectric layer, but the increase in thickness of the dielectric layer reduces direct-current field strength, which causes problems of decreases in a rate of capacitance change and in a control width of the impedance, resonant frequency, or the like, of the circuit.
With a high frequency signal, the variable capacitance element carries an electric current more easily and therefore, in the case of using the variable capacitance element in the variable capacitance circuit, loss resistance may cause the variable capacitance element to generate heat and be broken during use, which causes a problem that the circuit has lower power handling capability to the high frequency signal. To deal with such a problem, it is also effective to increase the thickness of the dielectric layer and reduce a calorific value per unit volume, but the increase in thickness of the dielectric layer decreases the direct-current field strength, which causes problems of decreases in the rate of capacitance change and in the control width of the impedance, resonant frequency, or the like, of the circuit.
JP-A 2005-101773 discloses a circuit having a variable capacitance element unit in which a plurality of the variable capacitance elements having thin-film dielectric layers with dielectric constant varying with a direct-current voltage are connected in parallel with respect to direct current and in series with respect to high frequency. In the circuit, because the plurality of the variable capacitance elements are connected in parallel with respect to direct current, a predetermined direct-current voltage can be applied to each of the variable capacitance elements, which allows for desired impedance control by taking full advantage of the rate of capacitance change of each of the variable capacitance elements caused by the direct-current voltage. In addition, because the variable capacitance element unit has the plurality of the variable capacitance elements which are connected in series with respect to high frequency, the high frequency voltage to be applied to the variable capacitance elements is divided for the respective variable capacitance elements, with the result that the high frequency voltage applied to each of the variable capacitance elements receives the divided and thus reduced voltage, allowing for reduction of the capacitance variation for the high frequency signal in the variable capacitance element unit. Hence, in the circuit disclosed in JP-A 2005-101773, it is possible to reduce waveform distortion, intermodulation distortion, or the like, of the circuit. Furthermore, because the plurality of the variable capacitance elements are connected in series with respect to high frequency, the same effect is obtained as that obtained by increasing the thickness of the dielectric layer of the variable capacitance element, and the calorific value per unit volume due to the loss resistance of the variable capacitance element unit can be smaller, thus allowing for enhancement of the power handling capability of the circuit.
However, on the other hand, the variable capacitance element unit in the circuit disclosed in JP-A 2005-101773 has a large number of variable capacitance elements, therefore allowing for reduction of the high frequency voltage applied to each of the variable capacitance elements, but in this case, the large number of variable capacitance elements contributes to the tendency to increase the current consumption owing to the fact that the variable capacitance elements are connected in parallel with respect to direct current, and furthermore, when a large number of the variable capacitance elements are used, the variable capacitance circuit tends to grow in size and cost.
The increasing number of variable capacitance elements can reduce the waveform distortion, intermodulation distortion, or the like, of the circuit while increasing the current consumption, and among the demand characteristics of a circuit having the variable capacitance element unit, the reduction of distortion and the reduction of current consumption have an antinomic relation and it is therefore hard to satisfy both of these in particular, with the result that no satisfactory report has been made about a variable capacitance circuit which even achieves sufficient controls on the increases in cost and size of the variable capacitance circuit attributable to the increase in the number of variable capacitance elements.